Phase 6d: systemd user unit for persistent FUSE mount

Deploy artifact — not built into the binary tree. `deploy/systemd/
clawstor-fuse.service` + README with the install recipe.

Unit shape:
* Type=simple, blocks on the FUSE binary (unmount = SIGTERM).
* ExecStartPre = mkdir -p mount, best-effort lazy unmount of any
  stale prior mount (guards against a hard SIGKILL leaving the
  kernel with a dangling mount).
* Restart=on-failure, RestartSec=5 — recover from transient IO
  errors without operator involvement.
* Environment= for CLAWSTOR_FUSE_BIN / DATA / MOUNT so a
  `systemctl edit` drop-in retargets without editing the unit file.

Dependency chain: clawstor-fuse.service After= + Wants=
clawstor-cluster.service. FUSE reads only the on-disk state so
this is technically not needed for correctness — but keeps the
mount from spinning up on a node where the daemon is broken.

deploy/systemd/README.md

@@ -0,0 +1,43 @@
+# deploy/systemd
+
+Systemd **user** units for clawstor.
+
+## Units
+
+| Unit | Purpose |
+|---|---|
+| `clawstor-cluster.service` | daemon (gossip + RPC + Prometheus + build cache). Not shipped here — deployed per-node from the fleet playbook. |
+| `clawstor-fuse.service` | Phase 6 read-only FUSE mount at `~/clawstor-mount/`. Depends on `clawstor-cluster.service`. |
+
+## Install `clawstor-fuse.service`
+
+Prereqs: `libfuse3` installed, `claw-fuse` binary built with
+`cargo build --release --features fuse --bin claw-fuse`.
+
+```bash
+# 1. Copy the binary
+cp target/release/claw-fuse ~/clawstor-deploy/claw-fuse
+
+# 2. Install the unit
+mkdir -p ~/.config/systemd/user
+cp deploy/systemd/clawstor-fuse.service ~/.config/systemd/user/
+
+# 3. Reload + enable
+systemctl --user daemon-reload
+systemctl --user enable --now clawstor-fuse.service
+
+# 4. Verify
+systemctl --user status clawstor-fuse.service
+ls ~/clawstor-mount/     # blobs  snapshots  tags
+```
+
+Override the default paths with a drop-in:
+
+```bash
+systemctl --user edit clawstor-fuse.service
+```
+
+```ini
+[Service]
+Environment=CLAWSTOR_MOUNT=/mnt/clawstor
+```

deploy/systemd/clawstor-fuse.service

@@ -0,0 +1,29 @@
+[Unit]
+Description=Clawstor read-only FUSE mount (blobs / snapshots / tags)
+Documentation=https://git.redclaw.dev/clawverse/clawstor
+After=clawstor-cluster.service network-online.target
+Wants=clawstor-cluster.service
+
+[Service]
+Type=simple
+# Override via `systemctl --user edit clawstor-fuse.service` when the
+# install lives elsewhere.
+Environment=CLAWSTOR_FUSE_BIN=%h/clawstor-deploy/claw-fuse
+Environment=CLAWSTOR_DATA=%h/clawstor-deploy/data
+Environment=CLAWSTOR_MOUNT=%h/clawstor-mount
+# Create the mount point if missing (idempotent). ExecStartPre runs
+# BEFORE the actual mount; fusermount3 needs an existing dir.
+ExecStartPre=/bin/mkdir -p ${CLAWSTOR_MOUNT}
+# Best-effort pre-unmount in case a prior instance left a stale
+# mount (e.g. daemon SIGKILL). fusermount3's `-u -z` = lazy detach,
+# harmless when nothing is mounted.
+ExecStartPre=-/usr/bin/fusermount3 -u -z ${CLAWSTOR_MOUNT}
+ExecStart=${CLAWSTOR_FUSE_BIN} --data-dir ${CLAWSTOR_DATA} --mount ${CLAWSTOR_MOUNT}
+# The FUSE binary blocks until unmounted. On systemd stop, send
+# SIGTERM which fuser translates to a clean unmount.
+ExecStop=/usr/bin/fusermount3 -u ${CLAWSTOR_MOUNT}
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=default.target